The present invention relates to a protective relay system for an electric power system and a sampling synchronizing method for synchronizing the sampling of electrical quantity at a plurality of terminals.
A protective relay system is known in which electrical quantity, such as electric current, is sampled at a plurality of positions or terminals in an electric power system, such as a power transmission system, and electrical quantity data in digital form obtained by the sampling is exchanged between the terminals and used for calculation for the purpose of the protective function. In such a system, the sampling at the respective terminals must be synchronized, i.e., the time at which the electrical quantity is sampled at the respective terminals must be made to coincide accurately, to ensure that electrical quantity data obtained by sampling simultaneously is used for the calculation.
There have been proposed various methods for synchronizing the sampling. In a method disclosed in Japanese patent application laid-open (Kokai) No. 110716/1979, the time of the transmission of the digital signal with respect to the time of the sampling is controlled to maintain a predetermined relation to the time of receipt of the digital signal from the opposite terminal with respect to the time of the sampling. A disadvantage of this system is relatively long delay of the transmission of the digital signal after the sampling. Such delay can become as long as almost one sampling period or interval. It is desirable that such delay be avoided particularly where quick-response protection of the electric power system is desired.
In another method which is disclosed in Japanese patent application laid-open (Kokai) No. 49645/1975, the time of transmission of a digital signal relative to the sampling is fixed or maintained constant, and the digital data transmitted contains cyclic additional data indicating indicium or number (1, 2, . . . . n). At each terminal, subsequent to reception of a digital signal of a certain number (i, 1.ltoreq.i.ltoreq.n), a digital signal of the identical number (i) is transmitted. The length of time from the transmission of the digital signal of a certain number to the subsequent reception of a digital signal of the same number is measured at both terminals, and the sampling timing is adjusted so that the above-mentioned lengths of time measured at the both terminals become equal to each other. A disadvantage of the system is that the amount of information or data to be transmitted is relatively large.